Regulatory T Cells
Studies have identified the existence of a naturally occurring population of regulatory/suppressor T cells, which, upon in vitro TCR-mediated stimulation, suppress the proliferation of effector T cells (von Herrath and Harrison, 2003; Allan et al., 2008; Brusko et al., 2008; Vila et al., 2009). These cells are central to the control of T cell homeostasis and in the modulation of immune responses to autoantigens, cancer cells, pathogens, and alloantigens.
In the periphery of young mice not prone to autoimmune diseases, regulatory T cells constitute a stable 10% of CD4+ T cells. This proportion appears to be reduced in mice genetically prone to autoimmune disease such as diabetes (Salomon et al., 2000). Transfer of regulatory T cells has been shown to prevent a wide range of experimental autoimmune diseases, including diabetes, experimental autoimmune encephalomyelitis, and colitis (Salomon et al., 2000; Wu et al., 2002; Kohm et al., 2002; Read et al., 2000). Furthermore, depletion of regulatory T cells has been shown to exacerbate various experimental autoimmune diseases, including collagen induced arthritis. In humans, an analogous population of CD4+CD25+ regulatory T cells has been identified in the peripheral blood and the thymus (Jonuliet et al., 2001; Annunziato et al., 2002).
Autoimmune Diseases
Many autoimmune disorders arise when cells of specific tissues become the targets of T lymphocytes (for reviews see Santamaria, 2001; Vila et al., 2009). Much of what is currently known about effector pathways of autoimmunity has been learned from spontaneous and experimental models of autoimmune disease. Type 1 diabetes mellitus (T1D) in non-obese diabetic (NOD) mice is a prototypic model of spontaneous, organ-specific autoimmunity. NOD mice spontaneously develop a form of autoimmune diabetes, closely resembling human T1D, that results from destruction of the pancreatic β-cells by T lymphocytes.
Studies of CD8+ T-cell-deficient NOD mice indicate that the initial β-cell insult in T1D is effected by cytotoxic CD8+ T cells. Several transgenic models of T1D have shown that CD8+ T cells can readily kill β-cells expressing transgenic neo-antigens; however, little is known about the antigenic specificity or specificities of the CD8+ T cells that kill β-cells in NOD mice. Wong et al. (1999) have reported that there is a CD8+ T-cell subpopulation that recognizes an insulin-derived peptide in the islets of young NOD mice. Furthermore, immunopathological studies of pancreata from human diabetic individuals and pancreas isograft recipients have suggested that destruction of β-cells in human T1D may also be effected, at least in part, by CD8+ effector T cells (Bottazzo et al., 1985).
Experimental autoimmune encephalomyelitis (EAE) is a prototypic experimental autoimmune disease that models human multiple sclerosis and that develops in susceptible rodent strains after immunization with myelin basic protein, proteolipid antigen or myelin oligodendrocyte protein (MOG). Evidence suggests that CD8+ T cells have a role in disease progression and severity (reviewed by Goverman, 1999). Myelin basic protein is processed in vivo by the MHC class I pathway, and a MOG-derived peptide activates encephalitogenic CD8+ T cells in vivo. There is also evidence for clonal expansions of CD8+ T cells in active multiple-sclerosis lesions (Babbe et al., 2000).
Graft-Versus-Host Disease
Graft-versus-host disease is a multistep process. It has been shown that effector T cells play the pivotal role in the induction of the disease. During the ‘induction phase’ the effector T cells see alloantigen disparities and then become activated and clonally expand (the ‘expansion stage’). These T cells then release cytokines and possibly chemokines (for example macrophage inflammatory protein 1α), resulting in the recruitment of other cell types (macrophages, granulocytes, natural killer cells, etc.) in the ‘recruitment phase’. Finally, the T cells and the other cell types mediate the pathology associated with graft-versus-host disease (the ‘effector phase’) (for a review see Murphy and Blazar, 1999).
There has been emphasis on delineating the effector mechanisms of graft-versus-host disease. T cells and other cells primarily mediate their effector functions through either FasL, perforin-granzyme-B or TNF. The use of knockout mice has demonstrated a pivotal role for each of these pathways in the effector stage of graft-versus-host disease. FasL and perforin-granzyme-B appear critical for solid organ pathology whereas TNF appears to mediate the wasting/weight loss associated with graft-versus-host disease. TNF also appears to be induced, along with other cytokines, after conditioning (Hill et al., 1997)—demonstrating that cytokines elicited by either the donor or the recipient affect graft-versus-host disease. TNF-receptor knockout mice and the use of anti-TNF antibodies have been shown to be protective in graft-versus-host disease models (Speiser et al., 1997).
Cancer
In the past, attempts have been made to trigger the immune system to mount an efficient response against malignant cells. Despite significant and promising progress, such a response has yet to be fully attained and many immune based therapies have proved disappointing.
Numerous studies using in vitro cellular assays demonstrate that cytotoxic lymphocytes have the ability to kill tumour cells. The cancer patient also has increased concentration of circulating immune complexes, indicating the immune system is active, particularly against certain tumour antigens. The level of these immune complexes can increase with disease progression (Horvath et al., 1982; Aziz et al., 1998).
Regulatory T cells have been implicated in a subject's immune response to cancer (North and Awwad, 1990; Gajewski et al., 2009). As most cancer antigens are actually produced by the patient they are considered as “self” by the immune system. Upon the presence, and/or increased quantity, of tumour antigen the host's immune system mounts a response characterized by the production of effector T cells which target cells producing the tumour antigen. However, in many instances these effector T cells are recognized by the immune system as targeting the host's own cells, and hence a population of regulator T cells are produced to down-regulate the effector T cell population. Thus, the production of these regulator T cells limits the ability of the immune system to effectively remove cancer cells.
Degenerative Diseases
Whilst degenerative diseases such as Alzheimer's disease are not classically considered to be mediated by inflammation or the immune system, in some instances the immune system may play an important role in the degenerative process. In addition, it has become clear that the immune system itself may have beneficial effects in nervous system diseases considered degenerative. Immunotherapeutic approaches designed to induce a humoral immune response have recently been developed for the treatment of Alzheimer's disease. In animal models, it has also been shown that immunotherapy designed to induce a cellular immune response may be of benefit in central nervous system injury, although T cells may have either a beneficial or detrimental effect depending on the type of T cell response induced (Monsonego and Weiner, 2003).
Infections
More recently, regulator T cells have been shown to be involved in a subject's immune response to a viral infection. WO 02/13828 describes the production of regulator T cells during retroviral infection, and methods of treating such infections by down-regulating the regulator T cell population whilst maintaining the effector T cell population. Since WO 02/13828 was filed there have been a large number of studies which have identified a role for regulatory T cells in the progression of chronic retroviral infections. This includes studies on Friend retrovirus infection (Iwashiro et al., 2001), Feline immunodeficiency virus (Vahlenkamp et al., 2004), Simian immunodeficiency virus (Hryniewicz et al., 2006; Estes et al., 2007) and many studies on HIV (Weiss et al., 2004; Kinter et al., 2004; Lim et al., 2006; Nilsson et al., 2006; Kinter et al., 2007a; Kinter et al., 2007b; Lim et al., 2007; Cao et al., 2009). The role of regulatory T cells in the progression of chronic retroviral infections has also been the subject of many recent reviews including those by Vahlenkamp et al. (2005), Belkaid and Rouse (2005), Rouse et al. (2006) and Dittmer (2004).
Treatment of Diseases Involving Effector and Regulator T Cells
Taking advantage of regulatory T cells has been complicated by an inability to expand and characterize this minor T cell subset, a population of cells reduced even further in autoimmune-prone animals and patients. For instance, studies have suggested that it may be impossible to reverse ongoing autoimmune diabetes due to the autoreactive T cells becoming resistant to suppression during the active phase of the disease. Prior efforts to expand regulatory T cells ex vivo have not achieved clinically sufficient expansion, nor demonstrable in vivo efficacy. The low number of CD4+CD25+ regulatory T cells, their anergic phenotype and diverse antigen specificity present major challenges to harnessing this potent tolerogenic population to treat autoimmune diseases and transplant rejection.
WO 03/070270 describes the use of acute phase inflammatory markers in regimes for the effective treatment of HIV. These methods rely on at least partially “resetting” the immune system by a treatment such as HAART followed by the analysis of acute phase inflammatory proteins as markers for effector and regulator T cell expansion. The emergence of acute phase inflammatory proteins appears to be linked to effector T cell expansion, which occurs before regulator T cell expansion, and thus the patient can be treated with a suitable agent which allows the effector T cell population to be maintained whilst destroying, preventing the production of, or reducing the activity of, regulator T cells. In essence, upon withdrawal of HAART treatment it was considered that the patient's immune system would treat the re-emerging HIV particles as a new infection, and hence a new population of effector T cells would be produced.
Similar to WO 03/070270, WO 03/068257 relates to at least partially resetting the immune system, however, in this instance in the context of the treatment of cancer. Again, the treatment is focussed on the initial re-emergence of effector T cells following a reduction in tumour load through techniques such as surgery or the administration of anti-proliferative drugs.
Following from the advances described in WO 02/13828, WO 03/070270 and WO 03/068257, it was later surprisingly found that the immune system is cycling in many chronic disease states such as cancer, retroviral infections (both in WO 05/040816) and autoimmune diseases (WO 06/026821). Thus, it is not essential that the disease state be “reset” to be able to target the regulator T cells to effectively treat the diseases involving regulator T cells such as cancer. Despite the ground breaking advances described in WO 05/040816 and WO 06/026821, variations between individuals, variations in sample testing, and the complexity of the disease states make it difficult to manage the data to allow the routine targetting of the desired cell type on the first attempt. Thus, there is a need for mechanisms to increase the likelihood of effectively treating a disease in which the immune system is cycling in the initial attempt(s).